This invention relates to generator rotors and specifically, to the repair of generator rotors that have experienced fretting damage and/or cracks in the rotor teeth located at the butt joints of the axially aligned rotor wedges.
Conventional dynamoelectric machines, such as generators used with gas and steam turbines, employ forged rotors of magnetic material into which radial slots are machined for receiving the conductive turns of field windings which are interconnected such as to produce a desired magnetic flux pattern. Typically, included in such conventional rotor slots are creepage blocks at both the top and bottom ends of the slot as well as coil slot wedges for resisting the radially outward forces exerted on the windings when the rotor is operational.
The slot wedges, which are generally dovetail shaped, are used to maintain the copper coils in place while the rotor is spinning at, for example, 3600 revolutions per minute. In the prior art such coil slot wedges were normally 6 to 12 inches long with a number of such wedges being required for each coil slot, particularly in the longer rotors with high electrical ratings. In an effort to decrease the number of parts that are required for assembly as well as increasing the overall speed of such assembly, full length wedges have been used in certain applications. For other applications, tolerances preclude the use of full length wedges and, in these cases, two or more wedges are used in each rotor slot. Cracks have been found, however, in the radial slot walls of several generators, at the butt joint between adjacent rotor wedges, apparently due to fretting damage found at the ends of steel wedges. If allowed to remain in the rotor, these cracks can grow and potentially cause a catastrophic failure of the rotor.
In the past, repairs to cracked generator rotor teeth have involved removing the damaged rotor tooth material. These repairs, however, have not done anything additional to eliminate the future occurrence of additional fretting fatigue damage.
It is also known to use aluminum rotor wedges in certain generator rotors, but these have not previously been recognized in the art as useful for the elimination of fretting fatigue.
One prior solution to the fretting fatigue problem was to machine away the damaged tooth material and to replace the multiple original short steel rotor pole wedges with a single, full-length aluminum wedge. As already mentioned above, however, there are many generator rotors that cannot make use of full length wedges and require the continued use of plural shorter wedges with original dimensions.